Fix comments in srpregister test
Remove password from start_authentication method, since we don't need it
Reset A value from the class after we don't need it anymore
Raise SRPAuthenticationError when errcode != 422 too
Broaden the exception type to catch
Refactor methods and emit event when we know uid is not None
Remove unneeded username and password class members
Check if request content is None before doing anything
Granularize exceptions

Also, replace get_content for reqhelper.get_content for tests
Add tests for SRPAuth class